(a) Statement of the Invention
The present invention relates to chemically joined, phase separated self-cured hydrophilic thermoplastic graft copolymers and their preparation. The hydrophilic copolymers have good wet strength, rendering them useful in diverse applications, particularly in the field of biomedics, e.g., contact lenses and artificial organs.
(B) Description of the Prior Art
Considerable research and development has been devoted to obtaining hydrophilic polymers having sufficient wet strength to render them substantially water-insoluble. For example, one type of hydrophilic polymer having improved wet strength has been disclosed in U.S. Pat. Nos. 2,976,576 and 3,220,960 to Otto Wichterle et al and M. F. Refojo et al, Journal of Applied Polymer Science, Vol. 9, pages 2425-2435 (1965). These polymers are prepared by simultaneously polymerizing a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group such as a hydroxy group and cross-linking the monomer as it polymerizes with a polyunsaturated cross-linking agent such as ethylene glycol dimethacrylate. The amount of such cross-linking agent is usually less than about one mole percent of the monoester. Such polymerizations are generally conducted in the presence of a redox initiator.
Hydrophilic polymers having improved wet strength derived from a polymer obtained by copolymerizing a water-soluble vinyl monomer containing at least one nitrogen atom with a small amount of a bifunctional monomer have also been disclosed. These polymers have been suggested as being useful in various biomedical applications due to their compatibility with body tissues and/or mucosa. Some of these polymers are disclosed in U.S. Pat. Nos. 3,639,524 and 3,767,731 to Maurice Seiderman.
One of the factors necessary for the manufacture of these prior art hydrophilic polymers having improved wet strength is the use of a small quantity of a polyunsaturated cross-linking agent. Since the quantity of the cross-linking agent employed is generally very small, the amount can only be varied slightly. Accordingly, it is not possible to effectively tailor the final properties of the hydrophilic polymer by varying the amount of cross-linking agent. Various attempts to avoid this problem have been disclosed in U.S. Pat. No. 3,503,942 to Seiderman and U.S. Pat. No. 3,758,448 to Paul Stamberger.
In co-pending application U.S. Ser. No. 282,099, filed Aug. 21, 1972, the disclosure of which is incorporated herein by reference, there is disclosed the bold and pioneering discovery for preparing chemically joined, phase separated thermoplastic graft copolymers. These copolymers in essence are physically cross-linked (as opposed to chemically cross-linked) type polymers and have been referred to as "self-cured" or "self-reinforced" thermoplastic graft copolymers. This phenomenon is occasioned by providing a controlled dispersion of a macromolecular sidechain in one phase (domain) within the backbone polymer phase (matrix). Because all of the macromolecular monomer sidechain domains are an integral part or interposed between large segments of the backbone polymer, the resulting graft copolymer possesses the properties of a cross-linked polymer, if there is a large difference in the Tg or Tm of the backbone and the sidechain segments. This is true up to the temperature required to break the thermodynamic cross-link of the dispersed phase. In order for the domains of the dispersed phase of the graft copolymers to impart the desired cross-linked or "self-curing" effect, it is important that the macromolecular monomers which comprise the domains of the dispersed phase have substantially the same molecular weight, i.e., the macromolecular monomers must have a Mw/Mn ratio which is not substantially above about 1.1. Macromolecular monomers having a broader molecular weight distribution, i.e., a ratio greater than about 1.5 or 2, will ordinarily possess polymeric species of very low molecular weight which impart a "copolymeric" effect on the backbone polymer and polymers of very high molecular weight which will form a second dispersed phase or domain having a size different than the primary dispersed phase. The end result is an opaque polymer of inferior quality with respect to the desired physical properties possessed by the backbone polymer.